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6au8a amplifier repair

Detrola » Jun Sat 12, pm. Users browsing this forum: Majestic [Bot] and 35 guests. Ads are not permitted. This is just some preliminary brainstorming. Any opinions on using a 6CX8 or 6JL8 for a replacement? The triode section can be used as a buffer stage following the LO, which is one reason I'm thinking about doing this, besides the ever increasing prices for 12BY7 tubes and their short life span in the SX

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Sales in the past four months have increased beyond the most optimistic expectations. The fall viewing season finds the three major networks devoting a majority of their prime time to color telecasts. What does this mean to you? It means a greater necessity for technicians qualified, to service color receivers. These pages will bring you up-todate on new circuits in the models, and also review briefly some of the circuits that have been retained from previous models.

First, though, let's have a look at some of the picture sizes. Luminance Channels There have been some refinements in at least one case a major overhaul-of the circuits handling the luminance, or Y, signal. Here's what the various manufacturers have done. The circuits are basically unchanged in the new Admiral receivers; however, some new tube types appear.. The G 11 chassis uses the pentode section of a 6BN11 compactron as the first video amplifier; the second video -amplifier stage uses the pentode section of a 10 -pin decal -type tube, a 6Y9.

The circuit shown in Fig. During a monochrome telecast, the video information is simply applied to the grid of the second video stage, amplified, and coupled to die third stage. During color reception, the signal on the grid of the bandpass amplifier causes it to conduct, and its plate current flows through R and R2. This lowers the potential at the junction of R1 and R2 and allows the neon light to fire, -and 1 P!

Tanner indicating color is being received. When the color -indicator light fires, the diode becomes forward biased, and the 3. These frequency components are thus prevented from appearing in the output of amplifier V 1. Two tubes and one transistor are used in the video -amplifier stages of General Electric's CB chassis.

The first stage uses the triode portion of a 6AU8A. The NPN transistor used in the second stage provides a low impedance that allows matching its collector output to the low input impedance of the delay line. Output of the delay line is fed to a 12GH7 used as the video output. Magnavox; Motorola, Packard Bell, Setchell Carlson, and Sears are using circuits almost identical to those found in the '65 models. These circuits, with the exception of the video driver, are similar in operation to those found in black -and -white receivers.

The video -driver stage, however, requires a closer look. The first and second stages are shown in Fig. Second-class postage paid at Indianapolis, Indiana. Current single issues 50e each; back issues 65c each.

Color indicator light is controlled by bandpass amp. The luminance signal is taken from the first -stage cathode. Its amplitude is adjusted by the setting of the contrast control, and it is then applied to the emitter of the video driver transistor. This stage is a common -base amplifier, and the bias is controlled mainly by the divider resistors in the base circuit. With the -ohm collector load resistor R1 , the output of the stage is matched to the delay line. L 1 and L2 are peaking coils which affect primarily the high -frequency signal components.

Additional high frequency peaking is provided by the low value pf of the base bypass capacitor. Output termination of the delay line is provided by R2. The most notable change in Sylvania's receivers is the two -stage COL. Transistor serves as video video -IF strip used in the D03 chassis.

The other two chassis in the '66 line both use a conventional three-stage IF. In the CTC 17X chassis, more complete blanking of the picture tube during vertical retrace is accomplished by using a transistor in the vertical -blanking stage.

This transistor is actually a switch that causes increased conduction of the second video -amplifier stage during vertical retrace. Increased conduction causes a more positive signal to be applied to the CRT cathodes, insuring cutoff of the picture tube.

When signal tracing in the second or third video -amplifier stages, remember that the composite video waveform, especially the vertical sync portion, is altered somewhat. The '66 Zenith receivers also incorporate improved blanking circuits. This covers what has happened since a year ago to the design of circuits handling luminance information. Now we move along to the chroma stages. Bandpass Amplifiers A two-stage bandpass amplifier circuit is used in all three of Admiral's new chassis.

Notice in Fig. We'll go more deeply into how this is accomplished a little later. When color isn't being transmitted, the second bandpass stage is cut off by a negative voltage applied to the grid from the color -killer plate. Automatic color control ACC seems to be popular in the new chassis.

The automatic color-control detector operates in the same manner as the killer detector used in earlier models, but the ACC controls the bandpass amplifier as well as the color killer. Here is how the ACC voltage is developed and how it affects the conduction of the bandpass amplifier.

When a burst signal is applied to the primary of T1, signals of opposite phase are present on the anode of X1 and the cathode of X2. A sample of the 3. When X 1 conducts, current passes through R 1, the common grid resistor for the color killer and bandpass amplifier.

X2 will not conduct as long as a relatively strong burst signal is present, because nearly the same potential is maintained on both the cathode and anode. When the amplitude of the burst signal decreases, conduction through X1 decreases; the voltage drop across R l is lowered, and in turn the bandpass-amplifier bias decreases.

This causes increased conduction in the bandpass stage when a weak color signal is being received. When no burst signal is present, diodes X1 and X2 conduct on opposite half-cycles of the oscillator signal, and equal but opposite current pulses pass through R 1. The result is zero DC voltage across R1 - 3. ACC voltage governs gain of bandpass it overcomes the cutoff bias and allows the tube to conduct.

Diode X1 prevents the grid of the second bandpass stage from becoming positive, because it conducts if its anode is more positive than its cathode. Cl and on the grid of the color killer.

When the color -killer tube conducts, it cuts off the second bandpass-amplifier stage. Sylvania's D01 and D02 chassis also have an automatic chroma level -control circuit.

Although a 6JU8 is used as the killer detector, the circuit operation is similar to that just described. During monochrome reception, the second bandpass amplifier is cut off by a negative voltage derived from the grid of the horizontal oscillator. When a color signal is received, an ACC voltage is developed and causes a more negative voltage to be present on the grid of the first bandpass amplifier. This increased negative voltage causes the first-stage screen voltage to increase considerably.

However, Chassis G 11 does have a refined color -sync circuit; it is shown in Fig. The 3. This oscillator functions whether or not a color program is being received, but during color reception, it is synchronized with the burst signal coupled through transformer L1. Bias voltage for the color killer and color -control voltage for the first bandpass-amplifier stage are developed by diode X1. When a burst signal is present, the diode increases conduction, and the negative DC voltage at the junction of R1 and Cl also increases.

This increased voltage biases off the color killer and controls conduction of the first bandpass-amplifier stage in accordance with the strength of the incoming color signal.

In other words, the weaker the burst signal is, the harder the bandpass stage is allowed to conduct. The color -sync circuit shown in Fig.

In this circuit, the familiar oscillator tube has been omitted-the 3. Here's how this is accomplished. With no input, the tube is cut off, and L2 and C3 act as a series -resonant trap holding the grid at ground potential for 3.

When a positive pulse from the horizontal -output transformer is applied to the resonant circuit composed of L1 and Cl, this circuit produces a damped train of oscillations at approximately 90 kc. The first positive half-cycle in this train is the only one of sufficient amplitude to fire neon bulb M1 When this bulb fires, a square wave is developed across R1 and causes the burst gate to conduct. During this period of conduction, the burst signal from the bandpass amplifier is applied to the cathode of the burst gate.

This signal, when amplified and applied to the high -Q resonant circuit consisting of L3, X1, and C6, causes it to ring at 3. Since the tuned circuit is excited only during horizontal -retrace time, the 3. The amplitude of the signal applied to the grid of the subcarrier amplifier is adjusted by L4. The tube functions as a limiter. Magnavox, Motorola, Packard Bell, Sears, Setchell Carlson, Sylvania, and Zenith have simply carried over their color -sync circuits from previous models.

Philco has a basically unchanged circuit in its 16QT85 and 17KT50 series. The minor modifications include making the Z -demodulator phasing coil tunable, changing the chroma phase detector and killer to a pair of diodes, and changing the burst amplifier from a 6EW6 to a 6GH8A.


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There appear to be various names for the phase splitter topology I used; It is variously known, depending on which text you're reading at the time, as; the Concertina phase splitter the Cathodyne phase splitter the Split Load Inverter This type of phase splitter and there are several alternative topologies , is cheep, easy to implement, and capable of good signal voltage balance between the two anti-phase outputs. Amongst it's disadvantages are the absence of voltage gain, and limitations to signal voltage swing, and relatively high output impedance, with the potential to significantly misbehave should the subsequent output valve s be over driven into grid current. An intermediary stage, between each of the two phase splitter anti-phase outputs, and the grids of the following output stage valves, is regularly seen in the literature; the Williamson circuit is a classic example. Such a stage is added to reduce the loading on the phase splitter itself, confer some additional voltage gain, and isolate the phase splitter from possible variations between anode and cathode circuit loads. I elected not to add such an additional stage, but to try to drive the phase splitter reasonably hard that is, with a high quiescent current , instead.

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Results for "tube tester" in All Categories in Ontario Showing 1 - 40 of 41 results. Notify me when new ads are posted. Sort by Posted: newest first Posted: oldest first Price: lowest first Price: highest first Best match. Repairs To Guitar Amps and More. Repairs to tube Leslies, guitar amps, Hammond organs, and more. Specializing in vacuum tube amp repair and updating old caps. Also repairs to guitar electronics. New bias output tube tester to accurately set up amps correct tube bias. Free estimates and low rates. Pick up and delivery if required.

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6au8a amplifier repair

Order by:. Available to:. Free Shipping. One 5r4gb GE Nos Nib tube. I have Two available and I combine shipping.

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6AU8A - Information

Order by:. Available to:. Free Shipping. Inline fuse holder, 5 x 20mm. These have been tested as new on a Hickok A by my amp tech.

During color reception, the signal on the grid of the bandpass amplifier causes TRANSFORMER BURST GATE 0 TO DEMOD 6AU8A Rl Ml R3 SUBCARRIER AMPLIFIER C2.

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This seller is situated in the United States, and started online selling on eBay on Jun 1, They operate a store called Power Grid Tubes. Until now, they have over one hundred items for sale. This does not affect your purchases or the price you may pay. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network. If the item you received is damaged, not as described in the item listing, or it is not the item you purchased, you can return the item to Powergridtube for a refund.

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About 12ax7 6j1 Tube Vs. This makes the power section work harder and can lead to a more distorted It is currently Dec Tue 14, am. Posted by 6 years ago. The type known as a thermionic tube or thermionic valve utilizes thermionic emission of electrons from a hot cathode for fundamental electronic functions such as signal 12BH7. Add to Cart Add to Cart.

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